horn_of_plenty-to-container #tip

For tipping purposes, a horn of plenty is treated like a container.
But using one as the source container in a container-to-container tip
wasn't supported.  Implement that.

Also, #tip was offering carried bags of tricks as candidate containers
to tip some other carried container into.  Only do that for ones which
aren't known to be bags of tricks (so when type not discovered yet, or
specific bag not seen yet due to blindness).

src/apply.c

@@ -4115,7 +4115,7 @@ doapply(void)
         res = do_play_instrument(obj);
         break;
     case HORN_OF_PLENTY: /* not a musical instrument */
-        (void) hornoplenty(obj, FALSE);
+        (void) hornoplenty(obj, FALSE, (struct obj *) 0);
         break;
     case LAND_MINE:
     case BEARTRAP: